PROJECT SUMMARY/ABSTRACT Intra-abdominal injury (IAI) and traumatic brain injury (TBI) are the two leading causes of death in children older than 1 year. Some IAIs and TBIs are difficult to identify, and failure to identify these injuries results in preventable morbidity and mortality. Abdominal and cranial computerized tomography (CT) scanning have become the reference standard for diagnosing IAI and TBI, and CT use has significantly increased over the last 30 years. CT scanning, however, has important risks, particularly the risk of radiation-induced malignancy. The risk of radiation-induced malignancy in young children is approximately one in 400 abdominal CT scans and one in 1,200 cranial CT scans. Currently, fewer than 10% of abdominal and cranial CT scans obtained in injured children demonstrate IAI or TBI, thus, CT scanning is used inefficiently. This inefficiency is primarily driven by physician concerns of missing injuries and the lack of rigorous evidence regarding indications for CT use after trauma. We have previously derived clinical decision rules for the use of abdominal or cranial CT scanning in injured children. These rules were derived in large, diverse, multicenter populations, and are robust and precise; however, these rules have not yet been externally validated in a large, multicenter setting. Appropriate validation of derived clinical decision rules is required before clinical implementation. Such validation should be performed in a large, diverse, multicenter population. The objectives of the current study are to validate these previously derived, highly accurate generalizable decision rules for abdominal and cranial CT scanning in injured children. Once validated, these decision rules will then provide the evidence for appropriate use of CT in injured children. We will additionally collect data on CT use in children who are very low risk for IAI or TBI by the decision rules. This information will then be used to assist in rule implementation. Implementation of these rules will allow for timely identification of children with IAI or TBI in need of intervention, while avoiding CT scanning in children with minimal risk of such injuries. The study will be a prospective, multicenter observational validation study of children with blunt abdominal or head trauma at one of six emergency departments in the national CTSA Emergency Care Translational Research Collaborative (ECTRC). The emergency departments of this consortium evaluate more than 300,000 children of diverse racial and ethnic backgrounds every year. More than 7,500 children with blunt abdominal and 20,000 children with blunt head trauma will be enrolled over 3 years at the participating centers. The variables in the previously derived decision rules will be collected prior to CT scanning such that validation of the decision rules can be performed in an unbiased fashion. Successful validation of these rules will enable implementation of the rules into clinical care and improve the care of injured children across the United States.